## Physics-SchoolUK.com

### The UK site for KS3 and KS4 Physics

#### KS3 3.3.4 Energy - heating and cooling

In this section we will learn about thermal energy and how it can transfer in just one or more of three different ways, and always from a place of high temperature to a place of lower temperature - never the other way round.

### Thermal energy of an object and its temperature

The thermal energy of an object is the quantity of energy stored in an object due to the vibration of its particles.

So, if one object has more particles than another object (ie it has a greater mass) and everything else is the same, then that object will store more thermal energy, eg a bath full of water at 60°C will store more thermal energy than a cup of water at the same temperature, 60°C. The proof of this is that you would find that the cup of water would lose its small amount of thermal energy and cool down much faster than the bath full of water.

If two objects are of the same material and mass but are at different temperatures, then the one at the higher temperature will store more thermal energy. I think that is pretty obvious.

A more subtle factor, however, is the material that objects are made from. Objects of identical mass and temperature but of different materials will store different amounts of energy. For example, blocks of iron and aluminium of the same mass and temperature, the aluminium block will store more energy. The same mass of water, at the same temperature, stores even more energy; a lot more.

So, the mass, temperature and material of an object will affect how much thermal energy it stores.

All 3 factors are important in assessing how much energy an object stores.

A little spark, even though its temperature is amazingly high, over 1000°C, stores far less energy than a bowl of water at about 50°C.

### Thermal energy TRANSFER due to temperature difference

Thermal energy will transfer from one place to another or from one object to another if there is a temperature difference.

The direction of thermal energy transfer is always from the high temperature to the low temperature.

If there is no temperature difference between places or objects then there will not be any thermal energy transfer.

Thermal energy will transfer between two objects or places until they are at the same temperature.

All of the above explains why a hot object always cools down; it will cool to the temperature of its surroundings eg to that of the room it is in (assuming the room is cooler than the object).

### How does thermal energy transfer?

Thermal energy will transfer from a hot object to a colder object or place in one of, or a combination of, 3 ways.

These ways are called:

• Conduction
• Convection

Conduction

The following are what we call "thought experiments"; we are not saying you should do them.

If a person touched the side of a cup or mug containing a hot drink and felt it warm, he/she would be experiencing thermal energy transfer from the hot drink through the side of the cup to your fingers by conduction.

If a person held one end of a metal rod with the other end in a flame, he/she would quickly feel the held end becoming warm and then hot. He/she would be experiencing thermal energy transfer from the hot flame through the metal rod to their hand by conduction.

However, if a person held a water filled boiling tube at the bottom such that the top of the tube was in a bunsen flame, as shown below, although the water at the top of the tube will eventually boil he/she would still be able to hold the bottom of the tube comfortably! This might seem amazing, but it shows that conduction does not occur in liquids.

Its difficult to suggest a thought experiment to show that conduction does not occur in gases, but if it doesn't occur in liquids, as we have already suggested, then it has no chance of occuring in gases.

From these simple thought experiments you can see that conduction requires a solid object through which the thermal energy can travel. It does not occur in liquids or gases.

Let's try to explain conduction

First, consider a material such as plastic or glass.
If we apply heat to one end of the material and if we could see inside the material, into its atoms, this is what we would see:

Particles within materials, such as atoms and molecules are always in motion. In a solid material like that shown above, they are always vibrating.
The hotter they are, the more they vibrate.
At the hotter end the atoms would be vibrating faster and further than those at the colder end.
This is shown in the diagram by the dashed lines around the blue particles.
So because they are vibrating faster and further (greater energy!) they bump into their neighbouring atoms and make them vibrate faster and further.
This progressive vibration gets passed on along the material until eventually all the atoms are vibrating at a similar amount, as shown below:

However, the conduction of thermal energy through a solid material (such as glass, plastic or ceramic like a mug) by this progressive vibration of particles is very slow.
In fact these materials are such poor conductors that we call them insulators of thermal energy.

Nevertheless, this explains why you do feel the (thin) sides of your mug of hot drink becoming warm (after a while) even though it is made from ceramic, an insulator of thermal energy.
Plastic and glass and ceramic of a reasonable thickness eg 1 cm, are good insulators.

If, however, we can space out the particles within a material such that the progressive vibration can't occur then we will really have very good insulators!
This is the case with liquids and gases where particles are sufficiently spaced apart that thermal energy does not get passed on by progressive vibration.
So, materials containing gases or liquids are the best insulators, eg polystyrene contains huge amounts of air pockets, so does fibre glass; these are very good insulators and are used in the insulation of buildings.

Is it possible to have very good conductors?

If we replace the bar of plastic or glass used in the explanation above for a bar of metal then our particle picture changes slightly but significantly.

As well as the regularly and closely spaced particles we now have free electrons which, as the name suggests, are free to move away from their parent particle and they will do so when given some thermal energy.

The arrows show the movement of the free electrons.
Starting at the hot end of the metal, the free eletrons will move rapidly through the metal bumping into other free electrons, passing on their energy until the thermal energy has spread evenly throughout the material; this happens very rapidly because these electrons are free to move.

So in a metal there are two mechanisms by which thermal energy is being conducted:
1. By the progressive vibration of the particles; this mechanism is slow, and
2. by the movement of free electrons throughout the metal; this mechanism is fast.

It is the 2nd mechanism that makes the metal a very good conductor.

To summarise:

Conduction is the transfer of thermal energy by the progressive vibration of particles and, in metals, by the movement of free electrons.

It relies on the closeness of particles, therefore conduction only occurs in solids, not in liquids or gases.

A thermal conductor: A material that allows heat to move quickly through it.

A thermal insulator: A material that allows heat to travel slowly through it.

Convection

When you see a large bird such as a sea gull flying without flapping its wings but rising higher and higher into the sky, it is rising up on a convection current; a rising current of warm air.

When you see a glider (an aeroplane without an engine) soaring gracefully like a bird, rising in circles, higher and higher, it is also making use of a rising current of warm air; a convection current.

When a hot radiator on one side of a room eventually spreads thermal energy to the whole room (not by conduction because that can't take place across an air gap), it has done so by means of convection.

When water is heated in a pan or a kettle with the heat source at the bottom, all the water in the pan/kettle eventually warms up and is brought to boiling point; the water is being heated by convection, not conduction.

Let's explain convection?

First of all notice from all the examples above that convection only occurs in gases such as air or in liquids like water. It does not occur in solids.

Gases and liquids together are known as fluids. So we can say, convection only occurs in fluids.

We will describe and explain convection using two examples, one with a liquid (water) and one with a gas (air).

1. Consider first a beaker of water:

A small amount of a purple dye has been put in the bottom to "colour" the water so we can see what is happening to the water just above the heat source:

As the water begins to heat up at the bottom of the flask, we would see the purple water rise up above the heat source:

Very quickly this hot water reaches the surface; it can't rise any further, of course, so it flows to the sides moving further from the heat source so it begins to cool down.
Since it has cooled, it begins to fall back down:

Eventually the falling, colder water, flows towards the heat source so it is reheated and rises again. We now have a complete current of rising and falling, hot and cold water; we call this a convection current.

Ok, we have described what happens, now we have to explain why it happens:

i. As the water at the bottom of the beaker is heated, the water particles/molecules gain energy.
ii. This extra energy causes them to move faster and further apart; they take up more space.
iii. So the heated water has increased in volume, becoming less dense than the colder water around it. (Remember Density = Mass/Volume, so if V increases whilst M remains unchanged, then D decreases.)
iv. Since this water is less dense than the surrounding water, it rises.
v. Colder water moves in to take its place and it is heated and rises so there is a column of rising warm water.
vi. Eventually as the warmed water moves further from the heat source it cools, becomes more dense and falls, joining the colder water at the bottom of the beaker, getting reheated, forming a continuous convection current.

2. Consider a convector heater (commonly known as a "radiator") in an air filled room:

The heater is mounted to a wall in the room.

Warm air rises above the heater; it cools as it moves away from the heater so it falls, flowing back to the heater to be reheated.

Thats our brief description of what happens; now for the explanation:

To explain what happens all we have to do is replace the word "water" from the previous example with the word "air".

i. As the AIR above the heater is heated, the AIR particles/molecules gain energy.
ii. This extra energy causes them to move faster and further apart; they take up more space.
iii. So the heated AIR has increased in volume, becoming less dense than the colder AIR around it.
iv. Since this AIR is less dense than the surrounding AIR, it rises.
v. Colder AIR moves in to take its place and it is heated and rises so there is a column of rising warm AIR.
vi. Eventually as the warmed AIR moves further from the heat source it cools, becomes more dense and falls, joining the colder AIR at the bottom of the room, getting reheated, forming a continuous convection current.

You should be able to see now why convection can NOT occur in a solid; it depends on the movement of the fluid (the liquid or the gas).

Also you should be able to understand the few examples mentioned at the start of this section eg why we heat water in a pan (or a kettle) at the bottom.

Finally, don't make the common mistake of saying "heat rises" when you mean "hot liquid (or gas) rises"; be careful to get the description correct.

To summarise:

Convection is the transfer of thermal energy when particles of a heated fluid (a liquid or a gas) rise.
It relies on the fluid movement of particles, so it does not occur in a solid.

The third and final way that thermal energy can transfer from object to object or place to place is by "radiation".
Note - do not confuse this use of the word "radiation" with its use in descriptions of Nuclear Radioactivity or Nuclear Radiation; our use of the word has nothing to do with nuclear radiation.

If you stand a few metres from an open fire, the heat that you feel on your body is heat that has transferred from the extremely hot fire to you by radiation.

If you stand outside on a clear sunny day, the heat that you feel on your body is heat that has transferred from the Sun across 93 million miles of space to Earth by radiation.

These two simple examples suggest that heat transfer by radiation is similar to the transfer of radiant light; this is correct - just as light travels at 300 million kilometres per second, so does thermal radiation. In fact, thermal radiation is part of the same Electromagnetic Spectrum of which visible light forms a part. They both travel as Electromagnetic Waves.

So they are very similar.
The difference, however, is whilst we can "see" visible light (hence its name), we can't see thermal radiation even though sometimes it travels alongside visible light as in the two examples above.

But most of the time we don't see thermal radiation travelling alongside visible light eg if you heated a few litres of water in a kettle then put your hand near to but not touching the side of the kettle (and definetly not above the kettle) you will feel some heat reaching your hand by radiation from the hot kettle, but you will not "see" any light.
Q. How do we know that this is due to heat transfer by radiation rather than by conduction or convection?
A. It is not due to conduction because we stated that conduction would not occur across an air gap. Its not due to convection because the hand is held away from the side of the kettle; if it had been held above the kettle then, yes, we would have felt the effect of hot air rising in a convection current.

So, what is transfer of thermal energy by radiation?

It is simply the transfer of thermal energy as a wave, an electromagnetic wave.

This makes transfer of thermal energy by radiation quite special and different to conduction and convection. For example, whilst they are both very slow moving processes, radiation is incredibly fast (as fast as light). Also, whilst they both involve particles, radiation travels as an electromagnetic wave which doesn't need particles and so thermal transfer by radiation can occur across a vacuum eg the vacuum of space, which is how we "see" the light and "feel" the heat from the Sun.

All objects transfer energy all the time by thermal radiation; the hotter the object the more energy it transfers each second.

All objects will radiate and absorb thermal radiation all the time (but not always at the same rate).

An object that radiates to its surrounding more than it absorbs from its surroundings will fall in temperature.

An object that absorbs from its surrounding more than it radiates to its surroundings will increase in temperature.

An object will absorb more from its surroundings if its surrounding are warmer than the object.

An object will radiate more to its surroundings if its surrounding are colder than the object.

Factors that make objects better or worse radiators/abosrbers

Apart from the temperature difference between the object and its surroundings which is the prime factor determining whether an object primarily radiates/absorbs, it turns out that the surface colour of an object plays a very significant part in determining whether an object is a good radiator/bad radiator or a good absorber/bad absorber. So whilst it is the temperature difference that will dictate exactly what happens, the surface colour of the object will dictate how well or badly the object performs!

We can investigate this via two experiments:
In the first experiment we look at how well objects emitt thermal radiation:

1. Take two hollow metal cubes of identical size, put them on heat resistant mats, fill each with same amount of recently boiled water and put a thermometer into each.
The only difference will be that one cube is left with its original shiny metal surfaces whilst the other has been painted on all sides (not top or bottom) with a matt black paint.

The user then takes temperature readings from each cube every 30 seconds.

Results:
A typical graph showing how the temperature of each cube varied over time would look like:

A you can see, the cube with the matt black sides cools down faster.

Conclusion: objects with matt black surfaces emitt thermal radiation faster than objects with shiny surfaces. They are better radiators of thermal radiation than shiny surfaces.

In the second experiment we look at how well objects absorb thermal radiation:

This time we take a single piece of metal and bend it into a U - shape as shown in the diagram below.
We paint the inside of one surface with matt black paint, leaving the other with its shiny metal surface.
In the centre of the U - shaped piece of metal we place a radiant heater.
On each of the two inside surfaces of the metal we attach a coin using some melted wax.

We turn on the radiant heater and watch as it warms up the two inside metal surfaces and begins to melt the wax.
We are watching to see, which coin falls down first? Is it the coin on the shiny metal surface or is it the coin on the matt black metal surface?

Result: The coin that was on the matt black surface slides down first; in fact, it slides down within about 30s of turning on the heater, whilst the coin on the shiny metal surface stays securely fixed for a "long" time; most people give up waiting for it to slide down!

Conclusion: Matt black surfaces are the best for absorbing thermal radiation.

Matt black surfaces are the BEST for radiating and absorbing thermal radiation.
Shiny metal surfaces (or even white surfaces) are BAD for radiating and absorbing thermal radiation. They are actually GOOD reflectors of thermal radiation.

So, what colour should you paint a room radiator if you really wanted it to heat the room to its best ability by thermal radiation? Answer - black!
So why are most room radiators painted white? It is for two reasons; one, they look better if they are white; two, despite their name, they predominantly heat a room by CONVECTION, not by thermal radiation. So it doesn't really matter what colour they are painted.

Why are shiny metal coloured cars very popular? Most likely it is just a fashion choice but it could be that they are most likely to remain cooer for longer on very hot summer days. The worst colour for a car is black.

When people travel to hot countries they take white or at least light coloured clothes, don't they, because white is better for reflecting thermal radiation so there is less chance that they will get too hot. The worst colour to wear in a hot country is black.

#### Keywords

Now that we have reached the end of this section we can focus on the keywords highlighted in the KS3 specification. You have already met each one, but it is important to learn them.

• Thermal conductor: Material that allows heat to move quickly through it.
• Thermal insulator: Material that only allows heat to travel slowly through it.
• Temperature: A measure of the motion and energy of the particles.
• Thermal energy: The quantity of energy stored in a substance due to the vibration of its particles.
• Conduction: Transfer of thermal energy by the vibration of particles.
• Convection: Transfer of thermal energy when particles in a heated ﬂuid rise.
• Radiation: Transfer of thermal energy as a wave